Often we need to measure light intensity for our experiments and we use this little gizmo to do the job – a LI-COR light meter. Measuring light intensity is quite complex, but I find this explanation from GroWell (Hydroponics & Lighting) to be very useful:
Photons are counted in micromoles (µmol). One µmol is 602 quadrillion photons! I can’t even imagine a quadrillion!
I took a measurement of outdoor light intensity in Glasgow in February – reasonably bright, fast moving clouds, occasional cloud cover, temperature around 4oC. As you can see there were around 160 x 602 quadrillion photons hitting the sensor per second.
When there was no cloud I got readings up to around 600 umol – light intensity really does rapidly change….moving to the shade of a building I recorded around 45umol
When we grow our plants in environmentally controlled growth cabinet we set the light intensity to around 130-150 mol. I suppose this is equivalent to a reasonably cloudy day in Glasgow! Plants will be used to much higher levels of light intensity though….how do they adapt to such rapid shifts in light?